As the use and complexity of digital circuits has increased in the last decade, so also has the need to test such circuits increased in order to ensure proper operation. Basically, two types of digital circuit test techniques have been developed, namely functional test techniques and so-called in-circuit test techniques.
In functional test techniques a known digital pattern is applied to the circuit input and the circuit output is compared to an expected response. Any differences between the actual and expected outputs provide an indication of improper circuit operation. Unfortunately, this technique only provides information regarding overall operation of a circuit, for example a circuit which has been assembled onto a printed circuit board. For failure diagnostic purposes it is very often desirable to test individual circuit elements or groups of elements which have been assembled onto a printed circuit board apart from the overall circuit.
In in-circuit testing techniques, testing is performed on a circuit element or elements isolated from the remainder of the circuit. In-circuit testing techniques generally involve the application of a preselected digital pattern to the input of an individual circuit element, a so-called device under test (DUT), followed by the comparison of the DUT response to an expected response. Since the circuit element or elements under test typically are connected to other circuit elements, it is necessary to overdrive any digital pattern or signal which is being applied by an "upstream" circuit element or logic device. Upstream logic devices are those devices whose outputs normally drive the inputs of the DUT. An overdrive signal is a signal which is superimposed at a selected location in a circuit.
In order to perform multiple simultaneous tests on several individual circuit elements mounted on a single printed circuit board, test devices such as that disclosed in U.S. Pat. No. 4,588,945 were developed. In such devices a printed circuit board having circuit elements mounted thereon is in turn mounted or affixed to a so-called bed-of-nails fixture. Each nail acts as an individual probe either providing a preselected signal to or receiving an output signal from a DUT. As described in that patent, a controller module applies multiple pregenerated signal patterns to multiple DUTs through a driver module. The DUT responses are received through a sensor module and compared to expected responses.
The present invention focuses on improvements to the driver module circuitry to be used for both the in-circuit testing of devices (such as that disclosed in U.S. Pat. No. 4,588,945) and for the functional testing of devices. A discussed in that patent, the driver module is made up of a multiplicity of identical driver circuits which generate the actual voltage signals provided to selected probes or nails in the test bed or bed-of-nails. These circuits each provide logic high, logic low and an "off" state, i.e. the so-called tri-state. Unfortunately, when combined with the necessary interface to the bed-of-nails, these driver circuits have experienced some problems during operation, in particular, the presence of so-called stair-stepping or ringing characteristics in the rising or falling edges of the output signal.
The ringing or stair-stepping problem with the driver signal manifests itself at the end of the transmission line which connects the driver circuit to the DUT. This ringing or stair-stepping is caused by the unterminated nature of the transmission line. In particular test situations, the problem is that the ringing or stair-stepping in the driver circuit signal will cause the signal to be misinterpreted by the DUT. For instance, if ringing or stair-stepping is present in the driver signal, the DUT may interpret such a phenomenon as so-called double clocking: detecting more than one edge or input when only one is expected. As a consequence, the DUT response may be substantially different than the expected response, even though the DUT may be an acceptable device. Thus, such ringing or stair-stepping would lead to a false determination of a bad DUT.